This application is related to commonly-owned, co-pending application entitled: IGNITER INTEGRATED LAMP SOCKET FOR HOT RE-STRIKE OF HIGH INTENSITY DISCHARGE LAMP [Ser. No. 12/166,548], filed simultaneously herewith.
This disclosure relates to discharge lamps, and more particularly to high intensity discharge (HID) lamps such as ceramic metal halide (CMH) lamps where an instant start, hot re-strike of the lamp is desired. It finds particular application in HID lamps that are replaceable relative to the socket assembly, although it will be appreciated that selected aspects may find application in related uses.
In an instant start, hot re-strike application, ignition voltage is typically on the order of tens of thousands of volts (e.g., 25-30 kilovolts). This high voltage necessitates the use of high voltage cable from the igniter to the lamp socket and consequently better electrical insulation. This, in turn, adds cost to the assembly. In addition, a high voltage cable radiates electromagnetic interference (EMI) by acting as an antenna during lamp ignition. This may serve to render hot re-strike impractical in EMI sensitive applications such as health care.
It is known in other fields, such as in the automotive discharge headlamp environment, to integrate an igniter, lamp socket, and lamp into a single unit. In those applications, if the lamp requires replacement, the entire lamp, igniter, and lamp socket are disposed of and are subsequently replaced since the individual components are an integrated assembly. However, in non-automotive applications, this is not economically feasible since the useful life of the rest of the components is not limited to the life of the HID lamp, and the additional cost to replace the entire assembly is deemed unnecessary. Thus a need exists for hot re-strike applications of HID lamps in non-automotive applications such as commercial lighting, display lighting, office, stores, museums, stage lighting, television and film studios, etc.
After being turned off, a standard HID lamp usually requires a cooling time varying from 5 minutes up to 15 minutes for the lamp to be turned on again. This cooling time is required because the pressure inside the HID lamp's arc tube could be tens of atmosphere when the lamp is hot. In other words, a typical or regular ignition voltage of less than 5 kilovolts which can start a cold lamp does not provide a strong enough electric field to reignite across a high pressure plasmic gap between spaced electrodes when the HID lamp is hot. In order to achieve a HID instant start re-strike of a hot lamp (i.e., a hot re-strike), the assembly requires a much higher ignition voltage. A greater than 25 kilovolt of ignition pulse was typically used in an automotive discharge headlamp, and greater than 30 kilovolts of ignition pulse was used in other hot re-strike HID lamps for specialty applications.
This high ignition voltage causes a lot of issues and extra costs. For example, the high ignition voltage causes an electrode tip to sputter which shortens the effective life of the electrode and eventually will cause lamp failure. The high ignition voltage also deposits electrode material on the wall of the arc, blocks the light from the light emitting plasma, and degrades lamp performance. Moreover, the insulation must be necessarily increased to prevent undesired arcing in a high ignition, instant start HID system. The increased insulation complicates lamp design and results in increased costs. The high ignition voltage also presents severe EMI interference issues so that the system designers must take extra precautions to shield an ignition pulse, particularly in certain industries where EMI cannot be tolerated. Further, the ignition voltage generator costs more in order to generate the higher ignition voltage.
Consequently, traditional approaches result in a much more expensive solution to achieve instant start. The dielectric breakdown voltage for air is approximately 3 kilovolts per millimeter. With an ignition voltage greater than 20 kilovolt level, a breakdown air gap increases from less than 2 millimeters for a standard HID lamp to greater than 7 millimeters for instant start lighting systems. As apparent, this results in significant costs to the instant start system in order to modify standard design by providing insulation for the higher ignition voltage, providing a high voltage rated lamp base and lamp holder, and also use of a high voltage cable required to transfer the ignition pulse. Thus, hot re-strike ignition pulses on the order of 25 kilovolts cause much more severe EMI interference and require extra effort to shield the system. A need exists for an improved solution for hot re-strike application of an HID lamp that is replaceable, and an assembly that is reliable, repeatable, and has reduced costs.